The electrical signals arising from the various pixels are collected during a phase of reading of the matrix and then digitized so as to be able to be processed and stored to form an image. The pixels are formed of a charge-collecting electrode, and of an electronic circuit for processing the electrical signal thus created. In a general manner, in each pixel, the collection electrode is associated with an electronic circuit, consisting for example of breakers, capacitor, resistors, and downstream of which is placed an actuator. The assembly consisting of the collection electrode and the electronic circuit makes it possible to generate electric charge and to collect the latter. Each pixel is linked to a reading circuit making it possible to read the amount of charge collected and to transfer it to a processing means. This type of radiation detector can be used for the imaging of ionizing radiations, and in particular X-rays or γ-rays, in the medical sector or that of non-destructive testing in the industrial sector, for the detection of radiological images.
In this case, the detector comprises a detector material, of scintillator or semi-conductor type, able to interact with incident radiation of X-ray or γ-ray type.
When the detector material is a semi-conductor, for example of the type CdTe, CdZnTe, HgI2, an interaction of an incident photon in the detector generates the creation of charge carriers of electron-hole type. In this case, each pixel comprises a collection electrode able to collect some of the charge carriers resulting from the interaction. One speaks of a direct-conversion detector, since the charge collected by the collection electrodes is created by the interaction of the radiation to be detected in the detector material.
When the detector material is a scintillator, for example an inorganic scintillator of the type CsI, NaI, LaBr3, an interaction of an incident photon generates a plurality of photons that are less energetic than the incident photon, whose wavelength is generally situated in the visible region. In this case, each pixel comprises a photodetector, for example a photodiode, which detects these photons and converts them into electric charge which is collected by a collection electrode. One speaks of an indirect-conversion detector since the charge collected by the collection electrodes does not arise directly from the interaction of the incident radiation in the detector material, but arises from the detection of the visible photons generated during the interaction.
Photon imagers, and in particular X-ray or γ-ray photon imagers, comprise a (semi-conductor or scintillator) detector material, coupled to pixels, the latter generally being disposed according to a matrix. The amount of charge collected by each pixel gives information on the location of the interaction as well as on the energy of the photons that have interacted in the detector.
Now, each interaction generates a large number of particles (photons in the scintillator case, electron-hole pairs in the case of a semi-conductor), which undergo a spatial dispersion in the detector material, and migrate toward various adjacent pixels.
The consequences of this dispersion are a degradation of spatial resolution, since several adjacent pixels are impacted, and a less precise estimation of the energy of the incident photon. One speaks of degradation of the resolution in energy.
To alleviate this problem, a solution has been proposed in U.S. Pat. No. 7,667,205. This solution consists in defining groups of pixels centered around a target pixel. Each group contains the target pixel and a few nearby pixels. A sum of output signals of each of the groups is calculated and these sums are compared by calculating differences between these sums. The group having the highest sum makes it possible to define the pixel that received the photon. Finally, the sum calculated for the group of this target pixel is assigned to the latter.
The applicants of the present patent application have realized that this solution exhibits a drawback. Indeed, the groups of two neighboring target pixels necessarily have common pixels. When comparing the sums of output signals of various neighboring groups, the values of the signals of the pixels in common cancel one another. The comparison may not take into account the pixel whose output signal is the strongest. This gives rise to inaccuracies in the location of the pixel that received the largest amount of signal. Additional steps are necessary to define the position of the interaction with more accuracy. Moreover, this gives rise to a large number of operations for processing the signals collected by each pixel, in particular the addition of the signals of the pixels of one and the same group, and then the subtraction of the signals of each group.